NSR 13.5 rail and BCM 16" upper. Something is crooked.

AR/M4 AR Technical Discussion
1

Today I assembled my new 16" LW BCM upper with a NSR 13.5 rail. Something is off center.

I can’t tell if it’s the rail or the barrel. When I look at it from the end, and from the top or bottom, it is definitely closer to the left side of the rail by several millimeters. It’s enough that I noticed it almost immediately after finishing the build.

I am leaning toward the rail being the culprit, because when looking down the barrel from the rear of the upper, I can’t see any sort of bias to the left at all. It appears perfectly straight. I did notice, however that the barrel was VERY hard to remove, much more so than previous uppers.

Two questions:

-How can I narrow the problem down to the barrel/receiver or the rail?

-Is this something that can be caused by improper torque on the barrel nut?

2

What torque spec did you use? What upper are you using? Does it have the index pin?

I would possibly remove the rail, slide it back into place and then use an optic mount like a TA-51 and clamp it to the upper and rail while you tighten the screws. down.

3

Sorry I should have specified that the rail is “clocked” perfectly to the upper, but the end of the barrel is horizontally biased towards the left side of the rail.

I used the proper torque as indicated in the rail installation instructions. 40ft-lbs three times followed by final torque of a max of 80 ft-lbs. There is an index pin, but it doesn’t specify how it is used. I am using a standard bcm upper receiver.

4

Could you post some pics?

5

I wrote about this a while ago.

Basically, the culprit is most likely your barrel extension or upper receiver.

The dimensions of those areas are not required by spec to be at the level of straightness that you need to ensure that your barrel will be perfectly centered inside a floating tube.

Still, it is not a functional issue, and if you can’t get over it you can fix it with some work.

Typos brought to you via Tapatalk and autocorrect.

6

Is this where those receiver truing tools come in?

7

Check to see if your gas block is touching the inside of the rail, or any rail additions you may have added.

I had a similar issue and found the gas block up against a mounted rail backing piece.

8

I too couldn’t tell if my BCM 14.5" middy had a crooked barrel, barrel nut or if the 12" C4 rails were just misaligned.

I solved it by buying a VTAC ALPHA 13" rail, and installing that…it was worse than the C4s.

I just returned it to BCM, so hopefully it’ll get fixed. With a pinned muzzle device, it’s important to me that whatever future rails I put on it (that work with a standard barrel nut) align well.

If it’s the UPPER, it would seem the barrel is also crooked right, or is it generally just the barrel nut?

(My post was deleted on TOS for fear of upsetting a sponsor…even though I prefaced it by saying I wouldn’t name the manufacturer.)

9

There is an index pin, but it doesn’t specify how it is used. I am using a standard bcm upper receiver.[/QUOTE]

the index pin is used with the VLTOR/Noveske upper

10

I had an NSR on my last Colt and didn’t use the pin–would have required drilling a hold in the upper. I never had a problem.

11

That’s what I was thinking.

This issue has been reported twice this week with new builds.

12

Has BCM been replacing the barrels in question, or are they saying “If the gun works, we’re not replacing anything.”?

13

We’ll see. Mine arrived there yesterday–waiting for a response/call or it to get shipped back. ::fingers crossed it gets fixed, not just realigned/shimmed::

14

How much was yours off by?

15

The 12" C4 rails were probably just 3/16"…maybe even 1/4" closer to the barrel on the left and 1/4" further on the other side–it was definitely visible, but when I held it in place and tightened it down, I could get it to where it was barely visibly noticeable–sticking my two forefingers, one would go in a bit, the other wouldn’t (so it was still off).
I have a feeling it might shoot itself out of alignment unless I used red loctite and/or a shim.

NOTE: The VTAC ALPHA 13"s were off even more…I wish I’d measured, but I’d say there was 33-40% more space between the barrel and the rail on one side.

I actually plan on running the VTAC until Fortis or someone comes out with a lightweight keymod that uses a standard barrel nut. I’ve now heard of 3 people with the same issue. Hopefully we’ll all get it “straightened” out.

16

Sometimes.

I’m not being intentionally cryptic, it’s just that it can be in a few different places, or with a tolerance stack that isn’t visible until 10" from the barrel nut.

If it’s the threads, there isn’t really anything you can do.
If it’s between the barrel extension and the face of the receiver, they can be “flattened”; the extent of which work can be anything between simple and soul-crushing.

The big thing to remember is that the TDP doesn’t control these dimensions as tightly as needed to have precise alignment of free-floated tubes and barrels; so you can have quality components all around and still have a “misaligned” barrel and tube. Frustrating, I know, but one of the results of advancing a design that was not specifically spec’d for that setup.

17

This is another guy, from TOS, who just put a Troy ALPHA on his BCM blem upper…my VTAC ALPHA was off exactly like this:

18

The problem there is mounting BUIS on the rig. You may or may not be able to adjust windage enough to even get it on paper. I had the same issue with a TRX Extreme on an unnamed upper. I was able to true that setup through judicious sanding of the interface and bridging the upper & forend top rail while torquing everything down.

And yes it was a pain in the ass. :frowning:

19

Yeah, for a $1200 new (non-blem) complete upper group, I shouldn’t have to do all that (in my opinion–we’ll see).

My hope is that they just replace it with a new one that is dead-center aligned.

20

My 4 year old BCM upper had the exact same problem. Regardless of the rail system/barrel nut, it was misaligned. It also had other issues, so I replaced the upper receiver, which took care of it.

When I originally purchased it from BCM, it also had a slightly canted FSB (to the left, when looking down the rifle while firing).